THE PRESIDENT: Mr. President, Mr. Secretary General, fellow leaders: For all the immediate challenges that we gather to address this week — terrorism, instability, inequality, disease

— there’s one issue that will define the contours of this century more dramatically than any other, and that is the urgent and growing threat of a changing climate.

Five years have passed since many of us met in Copenhagen. And since then, our understanding of climate change has advanced — both in the deepening science that says this once-distant threat has moved “firmly into the present,” and into the sting of more frequent extreme weather events that show us exactly what these changes may mean for future generations.

No nation is immune. In America, the past decade has been our hottest on record. Along our eastern coast, the city of Miami now floods at high tide. In our west, wildfire season now stretches most of the year. In our heartland, farms have been parched by the worst drought in generations, and drenched by the wettest spring in our history. A hurricane left parts of this great city dark and underwater. And some nations already live with far worse. Worldwide, this summer was the hottest ever recorded — with global carbon emissions still on the rise.

So the climate is changing faster than our efforts to address it. The alarm bells keep ringing. Our citizens keep marching. We cannot pretend we do not hear them. We have to answer the call. We know what we have to do to avoid irreparable harm. We have to cut carbon pollution in our own countries to prevent the worst effects of climate change. We have to adapt to the impacts that, unfortunately, we can no longer avoid. And we have to work together as a global community to tackle this global threat before it is too late.

We cannot condemn our children, and their children, to a future that is beyond their capacity to repair. Not when we have the means — the technological innovation and the scientific imagination — to begin the work of repairing it right now.

As one of America’s governors has said, “We are the first generation to feel the impact of climate change and the last generation that can do something about it.” So today, I’m here personally, as the leader of the world’s largest economy and its second largest emitter, to say that we have begun to do something about it.

The United States has made ambitious investments in clean energy, and ambitious reductions in our carbon emissions. We now harness three times as much electricity from the wind and 10 times as much from the sun as we did when I came into office. Within a decade, our cars will go twice as far on a gallon of gas, and already, every major automaker offers electric vehicles. We’ve made unprecedented investments to cut energy waste in our homes and our buildings and our appliances, all of which will save consumers billions of dollars. And we are committed to helping communities build climate-resilient infrastructure.

So, all told, these advances have helped create jobs, grow our economy, and drive our carbon pollution to its lowest levels in nearly two decades — proving that there does not have to be a conflict between a sound environment and strong economic growth.

Over the past eight years, the United States has reduced our total carbon pollution by more than any other nation on Earth. But we have to do more. Last year, I issued America’s first Climate Action Plan to double down on our efforts. Under that plan, my administration is working with states and utilities to set first-ever standards to cut the amount of carbon pollution our power plants can dump into the air. And when completed, this will mark the single most important and significant step the United States has ever taken to reduce our carbon emissions.

Last week alone, we announced an array of new actions in renewable energy and energy efficiency that will save consumers more than $10 billion on their energy bills and cut carbon pollution by nearly 300 million metric tons through 2030. That’s the equivalent of taking more than 60 million cars off the road for one year.

I also convened a group of private sector leaders who’ve agreed to do their part to slash consumption of dangerous greenhouse gases known as HFCs — slash them 80 percent by 2050.

And already, more than 100 nations have agreed to launch talks to phase down HFCs under the Montreal Protocol — the same agreement the world used successfully to phase out ozone-depleting chemicals.

This is something that President Xi of China and I have worked on together. Just a few minutes ago, I met with Chinese Vice Premier Zhang Gaoli, and reiterated my belief that as the two largest economies and emitters in the world, we have a special responsibility to lead. That’s what big nations have to do. (Applause.)

And today, I call on all countries to join us -– not next year, or the year after, but right now, because no nation can meet this global threat alone. The United States has also engaged more allies and partners to cut carbon pollution and prepare for the impacts we cannot avoid. All told, American climate assistance now reaches more than 120 nations around the world. We’re helping more nations skip past the dirty phase of development, using current technologies, not duplicating the same mistakes and environmental degradation that took place previously.

We’re partnering with African entrepreneurs to launch clean energy projects. We’re helping farmers practice climate-smart agriculture and plant more durable crops. We’re building international coalitions to drive action, from reducing methane emissions from pipelines to launching a free trade agreement for environmental goods. And we have been working shoulder-to-shoulder with many of you to make the Green Climate Fund a reality.

But let me be honest. None of this is without controversy. In each of our countries, there are interests that will be resistant to action. And in each country, there is a suspicion that if we act and other countries don’t that we will be at an economic disadvantage. But we have to lead. That is what the United Nations and this General Assembly is about.

Now, the truth is, is that no matter what we do, some populations will still be at risk. The nations that contribute the least to climate change often stand to lose the most. And that’s why, since I took office, the United States has expanded our direct adaptation assistance eightfold, and we’re going to do more.

Today, I’m directing our federal agencies to begin factoring climate resilience into our international development programs and investments. And I’m announcing a new effort to deploy the unique scientific and technological capabilities of the United States, from climate data to early-warning systems. So this effort includes a new partnership that will draw on the resources and expertise of our leading private sector companies and philanthropies to help vulnerable nations better prepare for weather-related disasters, and better plan for long-term threats like steadily rising seas.

Yes, this is hard. But there should be no question that the United States of America is stepping up to the plate. We recognize our role in creating this problem; we embrace our responsibility to combat it. We will do our part, and we will help developing nations do theirs. But we can only succeed in combating climate change if we are joined in this effort by every nation –- developed and developing alike. Nobody gets a pass.

The emerging economies that have experienced some of the most dynamic growth in recent years have also emitted rising levels of carbon pollution. It is those emerging economies that are likely to produce more and more carbon emissions in the years to come. So nobody can stand on the sidelines on this issues. We have to set aside the old divides. We have to raise our collective ambition, each of us doing what we can to confront this global challenge.

This time, we need an agreement that reflects economic realities in the next decade and beyond. It must be ambitious –- because that’s what the scale of this challenge demands. It must be inclusive –- because every country must play its part. And, yes, it must be flexible –- because different nations have different circumstances.

Five years ago, I pledged America would reduce our carbon emissions in the range of 17 percent below 2005 levels by the year 2020. America will meet that target. And by early next year, we will put forward our next emission target, reflecting our confidence in the ability of our technological entrepreneurs and scientific innovators to lead the way.

So today, I call on all major economies to do the same. For I believe, in the words of Dr. King, that there is such a thing as being too late. And for the sake of future generations, our generation must move toward a global compact to confront a changing climate while we still can.

This challenge demands our ambition. Our children deserve such ambition. And if we act now, if we can look beyond the swarm of current events and some of the economic challenges and political challenges involved, if we place the air that our children will breathe and the food that they will eat and the hopes and dreams of all posterity above our own short-term interests, we may not be too late for them.

While you and I may not live to see all the fruits of our labor, we can act to see that the century ahead is marked not by conflict, but by cooperation; not by human suffering, but by human progress; and that the world we leave to our children, and our children’s children, will be cleaner and healthier, and more prosperous and secure.